1. Field of the Invention
The present invention relates to laser shock processing, and more specifically, it relates to laser peening techniques for providing identification markings on metal and other compressible materials.
2. Description of Related Art
Mechanical systems often require some form of part marking to identify the specific component for replacement, safety evaluation, tracking etc. Painted or printed identification often cannot be used because it fades, burns or wears away. Numbers can be etched or laser marked (vaporized) into the surface but removal of material or melting of material can leave a condition of local stress that can become the initiator of a fatigue or stress corrosion crack that can lead to failure of the component. Identification marks can be stamped into a part leaving a compressive stress but the depth and intensity of the compression is unsatisfactory. A stamp is also not easily changed to uniquely mark individual parts.
Using high power lasers to improve material properties is one of the most important industrial applications of lasers. Lasers can transmit controllable beams of high-energy radiation for metalworking. Primarily, the laser can generate a high power density that is localized and controllable over a small area. This allows for cost effective and efficient energy utilization, minimizes distortions in surrounding areas, and simplifies material handling. Since the laser pulse involves the application of high power in short time intervals, the process is adaptable to high-speed manufacturing. The fact that the beam can be controlled allows parts having complex shapes to be processed. Also accuracy, consistency, and repeatability are inherent to the system.
Improving the strength of metals by cold working undoubtedly was discovered early in civilization, as ancient man hammered out his weapons and tools. Since the 1950s, shot peening has been used as a means to improve the fatigue properties of metals. Another method of shock processing involves the use of high explosive materials in contact with the metal surface.
The use of high intensity laser outputs for the generation of mechanical shock waves to treat the surfaces of metals has been well known since the 1970s. The laser shock process can be used to generate compressive stresses in the metal surfaces adding strength and resistance to corrosive failure.
Lasers with pulse outputs of 10 to 100 J and pulse durations of 10 to 100 ns when condensed to fluences of 50 to 200 J/cm2 are useful for generating inertially confined plasmas on the surfaces of metals. These plasmas create pressures in the range of 10,000 to 100,000 atmospheres and the resulting shock pressure can exceed the elastic limit of the metal and thus compressively stress a surface layer as deep or deeper than 1 mm in the metals. Lasers are now becoming available with average power output meaningful for use of the technique at a rate appropriate for industrial production.
In the process of laser shock processing, a metal surface to be treated is painted or otherwise made xe2x80x9cblackxe2x80x9d that is, highly absorbing of the laser light. The black layer both acts as an absorber of the laser energy and protects the surface of the part from laser ablation and from melting due to the high temperature of the plasma. A thin layer of water, typically 1 to 2 mm, is flowed over this black surface. The water acts to inertially confine or, as it is called, tamp the plasma generated as the laser energy is absorbed in the short time pulse duration, typically 30 ns. Other suitable materials that act as a tamper are also possible. A limitation to the usefulness of the process is the ability to deliver the laser energy to the metal surface in a spatially uniform beam. If not uniform, the highest intensity area of the light can cause a breakdown in the water which blocks delivery of meaningful energy to the painted metal surface. A conventional technique to deliver the laser light to the surface is to use a simple lens to condense the laser output to a power density of roughly 100 J to 200 J per square centimeter. This condensing technique has the limitation that a true xe2x80x9cimagexe2x80x9d of the laser near-field intensity profile is not obtained at the surface. Rather a field intensity representing something between the near and far fields is generated. Diffraction of the laser beam as it is focused down onto the surface results in very strong spatial modulation and hot spots.
Any phase aberrations generated within the beam, especially those associated with operation of the laser for high average power, can propagate to generate higher intensity areas within the beam. These high peak intensity regions cause breakdown in the water layer, preventing efficient delivery of the laser energy to the surface to be treated. Another potential cause of breakdown in the tamping material is the generation of non-linear effects such as optical breakdown and stimulated scattering. In a normal generation of a 10 ns to 100 ns pulse within a laser, the output slowly builds over a time period exceeding several pulsewidths. This slow, weak intensity helps to seed the non-linear processes that require buildup times of 10 s of nanoseconds. In conventional techniques, the pulse output of the laser is xe2x80x9cslicedxe2x80x9d by an external means such as a fast rising electro-optical switch or by an exploding foil. These techniques can be expensive and can limit reliability.
It would be desirable if a laser peening process could be used to provide identification marking on metals and leave the surface in a state of deep residual compressive stress.
It is an object of the present invention to provide a method and apparatus for marking compressible components such as those made of metal and plastic.
It is another object of the invention to provide a method and apparatus for producing marks in compressible components that leave the component in a state of deep residual compressive stress.
Other objects of the invention will be apparent to those skilled in the art based on the teachings herein.
The invention is a method and apparatus for marking compressible components. One embodiment of the laser peenmarking system rapidly imprints, with single laser pulses, a complete identification code or three-dimensional pattern and leaves the surface in a state of deep (1 mm to 2 mm depth) residual compressive stress. A state of surface compressive stress in metal and other parts is highly desirable to make them resistant to fatigue failure and stress corrosion cracking. Most current marking techniques such as dot peening and laser engraving leave tensile stress in the surfaces and hence increase the potential for failure. The high quality and unique style of the pattern marked by the laser peening process of the present invention makes the pattern difficult to copy and thus allows such pattern to function as a xe2x80x9cwater markxe2x80x9d for preventing counterfeiting. This process employs a laser peening system and beam spatial modulation hardware or imaging technology that can be setup to impress full three dimensional patterns into metal surfaces at the pulse rate of the laser, a rate that is at least an order of magnitude faster than competing marking technologies. An alternate embodiment utilizes low power laser pulses that peen individual elements that then make up the matrix or other mark
Marking in compressive stress leaves parts highly resistant to fatigue failure and stress corrosion cracking. The laser peening process generates a shock wave as intense as 106 psi that strains a metal surface in a two-dimensional pattern directly correlated to the laser intensity profile at the metal surface. By creating a desired pattern upstream in the light field and imaging this pattern onto the metal surface, the full desired pattern is rapidly printed with each pulse of the laser. By spatially modulating the near field intensity profile of the laser light, a three-dimensionally imaged pattern can be printed into the metal with each pulse of the laser.
A data matrix, comprised of an array of white and black squares can easily be programmed into a spatial light modulator and a chosen matrix printed on a workpiece. This matrix is becoming the accepted marking pattern for airplane and aerospace components. Of particular interest is the ability to print a binary matrix of raised and recessed spots that represents the new marking standard defined by the Aerospace Transport Association 2000 (ATA200). The binary code is formed as a matrix having a perimeter and data contained therein. The perimeter is provided with density indicia for indicating the density of data contained within the matrix. The perimeter is also provided with size indicia for indicating the size of the matrix. By utilizing the density indicia and size indicia, a scanning device is able to calculate the size and information density of the binary code.
The present invention utilizes laser peening to create an enormously greater surface pressure in a metal and other compressible workpieces than provided by stamping and drives a deep (1 mm to 2 mm) intense compressive stress into the workpiece. Beam spatial modulation enables an equivalent stamping of complete identification marks with each laser pulse. Processing can be done at a rate of 5 to 10 marks per second, which is a rate unmatched by any other system. This process will be especially important for marking of critical parts such as aerospace and airplane components.